Technology for extracting and imaging form or tissue degeneration with time of a target object, by using images with different time phases, is useful in therapy effect judgment or lesion discrimination or the like. For example, by correctly extracting and imaging cancer sizes before and after drug dosage, therapy effect of a drug can be judged at an early stage. In addition, use of the technology as a monitoring tool for less invasive therapy, wherein therapy sites cannot be viewed directly, is capable of detecting tissue change of target areas in high sensitivity, and providing therapy not to invade normal sites. Usually, change of a target object with time is judged by an operator, by comparative viewing of images or by superimposing images themselves to be compared, based on characteristic sites such as a skeleton or the like.
Use of time-series images is also capable of extracting a blood vessel structure by tracking movement of an ultrasonic contrast medium. The ultrasonic contrast medium is a minute air bubble with a diameter of about several μms, and irradiation of ultrasonic waves generates a strong nonlinear signal. Because the ultrasonic contrast medium administered from a vein infiltrates into a fine blood vessel with blood flow, a high contrast image can be obtained, wherein a blood vessel structure is highlighted. However, because concentration of the ultrasonic contrast medium is lowered in a blood vessel at a peripheral part, a signal with sufficient S/N ratio (ratio of a signal to noise) cannot be obtained. In addition, at deep areas, reduction of sound pressure of irradiated ultrasonic beams, or fading of a nonlinear signal from the contrast medium also reduces S/N ratio and weakens contrast. Therefore, an operator tracks flow of the ultrasonic contrast medium by eye and judges a blood vessel structure in his (her) head. As a tool to solve a problem of decrease in S/N ratio at deep areas, a method for dividing imaging areas in a depth direction has been proposed (JP-A-2002-209898). Because of irradiation of ultrasonic waves in sound pressure sufficient to obtain a contrast medium signal, for each of the areas with different depth, dispersion of irradiated sound pressure in a depth direction can be reduced. By joining up each of the images acquired, an image, wherein high S/N ratio is maintained, can be obtained even in deep areas.
According to a pulse inversion mode (U.S. Pat. No. 6,095,980), it has been disclosed that measurement of body movement by cross-correlation calculation using an RF data, and accumulation of images are possible, even in the case where change of a contrast medium signal is large. A pulse inversion mode is a method for acquiring higher harmonic wave components in high S/N ratio, by continuous irradiation of a fundamental wave and a reversed phase wave, and accumulating reflecting waves of both. For example, in the case where a fundamental wave, f0=reiθ, and a wave having a reversed phase thereto, f1=rei(θ-π), are irradiated, each of the reflecting waves thereof contains, in addition to waves of fundamental frequency components, f0re=reiθ, and f1re=rei(θ-π), high frequency wave components, f0re2=rei2θ, and f1re2=rei2(θ-π. By subjecting these to accumulating processing, fundamental frequency components disappear, and higher harmonic wave components double. On the other hand, by subjecting these to subtracting processing, only fundamental frequency components are left, and high frequency wave components disappear.